1. Field of the Invention
The present invention relates to a high-density magnetic recording medium and particularly to a magnetic recording medium used for a so-called linear magnetic recording/reproducing system in which signals are recorded and reproduced using a magnetoresistive magnetic head (MR head) or a giant magnetoresistive magnetic head (GMR head) while the magnetic head is moved in both directions along the longitudinal direction of a magnetic tape. The present invention also relates to a magnetic recording/reproducing system using the magnetic recording medium.
2. Description of the Related Art
In the field of video tape recorders and the like, so-called metal thin film-type magnetic recording media have recently been applied for achieving higher image quality and a higher recording density, the magnetic recording media having a configuration in which a magnetic layer is formed directly on a nonmagnetic support by a vacuum thin-film forming technique using a magnetic metal material or any one of various magnetic materials such as, a Co—Ni alloy, a Co—Cr alloy, a Co—CoO metal oxide, and the like.
Further, in order to improve the electromagnetic conversion characteristics of the above-described magnetic recording media and achieve higher output, so-called oblique deposition has been proposed for forming magnetic layers of the magnetic recording media. Magnetic recording media including magnetic layers formed by this method have been put into practical use as deposited tapes for high-band 8 mm video tape recorders and digital video tape recorders.
The above-described metal thin film-type magnetic recording media are excellent in coercive force and remanence ratio and permit the formation of very thin magnetic layers, and thus have excellent electromagnetic conversion characteristics in a short wavelength region and small recording demagnetization and thickness loss in reproduction. Unlike in so-called coat-type magnetic recording media including magnetic layers which are formed by applying to a nonmagnetic support a magnetic coating material containing magnetic powder dispersed in a binder, a binder as a nonmagnetic material is not mixed in the magnetic layers, and the packing density of a ferromagnetic metal material is increased, causing advantage in increasing the recording density.
In addition, magnetic tapes produced by oblique deposition are produced by a method of depositing a magnetic material on a main surface of, for example, a long nonmagnetic support under a condition in which the nonmagnetic support is run in the longitudinal direction thereof. In this case, high productivity and excellent magnetic characteristics may be secured.
On the other hand, the recording densities of magnetic recording media increase with the increasing demand for magnetic recording media, such as magnetic tapes, as data streamers. Further, magnetoresistive magnetic heads (MR heads) or giant magnetoresistive magnetic heads (GMR heads) are applied as magnetic heads for reproducing recorded information in stead of related art inductive heads. The MR heads and GMR heads are capable of detecting with high sensitivity small magnetic leakage flux from magnetic layers and are advantageous in improving the recording density.
The MR heads and GMR heads have a detection limit at which sensitivity to a magnetic leakage flux is saturated, and thus greater magnetic leakage flux than the design limitations of the MR heads and GMR heads may not be detected. Therefore, the sensitivity to magnetic leakage flux may be optimized by decreasing the thickness of a magnetic layer of a magnetic recording medium.
As a recording/reproducing system for a magnetic tape used as a data streamer, the two systems, i.e., a helical san system and a linear system, are put into practical use. The helical scan system is one in which a magnetic head disposed on a rotating drum scans on a magnetic tape while rotating at a high speed to record and reproduce signals.
The helical scan system is basically capable of control to permit precise recording on a recording track and precise scan on the recorded track during reproduction. Therefore, a high recording density may be achieved in a magnetic tape system. Such a helical scan system is put into wide practical use as a home video picture recorder such as VHS, a high-band 8 mm video tape recorder, and a digital video tape recorder.
On the other hand, the linear system is one in which tracks are provided on a magnetic tape in the width direction thereof so that recording and reproduction are preformed in the longitudinal direction. This system is capable of easily running a tape at a high speed and improving a transfer rate of recording and reproduction by arranging many magnetic heads in parallel.
The helical scan system capable of achieving a high recording density is advantageous for magnetic recording tape systems used for camcorders. However, the linear system is widely used for data storage application in which the volume of a magnetic recording tape system is not greatly limited. In the market, mainstream commercial products are DLT (digital linear tape) and LTD (linear tape-open).
As a magnetic tape medium for such linear system data storage application, only a so-called coat-type magnetic tape is used, and a magnetic tape medium produced by oblique deposition has been not used. This is because in the helical scan system, a magnetic tape and a magnetic head are relatively moved in a constant direction, while in the linear system, a magnetic tape and a magnetic head are relatively moved in both directions along the longitudinal direction of the tape.
FIG. 1 is a schematic sectional view of a magnetic tape medium produced by oblique deposition. As shown in FIG. 1, a magnetic layer 92 is formed on a nonmagnetic support 91. A magnetic tape medium produced by oblique deposition has a structure in which the easy magnetization axis along which recorded magnetic bits are oriented rises from a tape plane, not extend in a planar direction of the tape (lateral direction in the drawing).
When a head slides in the forward direction (direction of arrow A) on the columnar structure of an obliquely deposited film in recording and reproduction, good recording/reproduction characteristics are exhibited. However, when a head slides in the backward direction (direction of arrow B) on the columnar structure of an obliquely deposited film, characteristics such as the optimum record current, phase characteristics, CN ratio, and output characteristics are inferior to those in sliding in the forward direction, and good recording/reproduction characteristics are not exhibited.
Therefore, a magnetic tape medium produced by oblique deposition has rarely been used for the linear system in which recording and reproduction are performed in both directions. As a method for resolving the problem in which the recording/reproduction characteristics in sliding in the forward direction on the columnar structure of the obliquely deposited film are different from those in sliding in the backward direction, there has been proposed a method of forming two layers of obliquely deposited films with different growth directions to form a magnetic layer of an obliquely deposited tape (refer to Japanese Unexamined Patent Application Publication No. 4-53622). FIG. 2 is a schematic sectional view of a magnetic tape medium described in Japanese Unexamined Patent Application Publication No. 4-353622.
As shown in FIG. 2, a magnetic layer 102 is formed on a nonmagnetic support 101, the magnetic layer 102 having a structure in which a lower ferromagnetic metal thin film 102a and an upper ferromagnetic metal thin film 102b are laminated. The oblique columnar structures of the lower-layer ferromagnetic metal thin film 102a and the upper-layer ferromagnetic metal thin film 102b are grown in opposite directions in the longitudinal direction of the nonmagnetic support 101. The thicknesses of the lower-layer ferromagnetic metal thin film 102a and the upper-layer ferromagnetic metal thin film 102b are optimized to decrease a difference in recording/reproduction characteristics in both directions.
Japanese Unexamined Patent Application Publication No. 9-73621 discloses a magnetic recording medium including a laminate of two layers of obliquely deposited films having obliquely columnar structures grown in different directions. In the magnetic recording medium disclosed in Japanese Unexamined Patent Application Publication No. 9-73621, the ratio of maximum coercive force/minimum coercive force obtained by changing the applied magnetic field angle from 0° to 180° is 0.65 or more in order to improve the recording/reproduction characteristics in both directions.
Japanese Unexamined Patent Application Publication No. 2000-339605 discloses a magnetic recording method in which the thickness of a magnetic layer composed of a single layer of cobalt-based obliquely deposited film is controlled to be ½ or less of the gap length of a magnetic head, thereby permitting linear-system recording and reproduction. This method preferably uses a cobalt-based obliquely deposited film having a thickness of 40 nm or less and a coercive force of 1,800 Oe or more.
Japanese Unexamined Patent Application Publication No. 2004-326888 discloses that even in an obliquely deposited tape including a single magnetic layer formed by a vacuum thin-film forming technique and having an oblique columnar structure, when the ratio Hcmax/Hc0 is adjusted to 1.2 or less, wherein Hcmax is the maximum coercive force in a plane perpendicular to the magnetic layer and including the longitudinal direction of the magnetic recording medium, and Hc0 is coercive force in the longitudinal direction of the magnetic recording medium, the same characteristics may be exhibited in the forward and backward directions.
On the other hand, a coat-type medium exhibits the same characteristic values in the forward and backward directions even when produced by a usual coating method because a magnetic material is not arranged obliquely as long as a general production method is employed.
However, as described in Japanese Unexamined Patent Application Publication No. 6-251355, there has been proposed a method using, for longitudinal recording, hexagonal ferrite generally developed as a vertical medium. However, as described in paragraph [0072] in Japanese Unexamined Patent Application Publication No. 6-251355, the recording wavelength evaluated at a frequency of 7 MHz using a 8-mm video deck is estimated to be about 0.5 μm, and the composition may be unsuitable for a current digital magnetic recording system used at a shortest recording wavelength of 0.25 μm or less.
In addition, as described in Japanese Patent No. 3393491, there has been the idea that a vertical magnetic recording medium including a thin film formed by vacuum deposition is used for a ring head. However, as evaluated in comparative examples below, the vertical magnetic recording medium is unsuitable as a magnetic tape used in a linear serpentine system because of the large unbalance between the characteristics in the forward and backward directions.